Cannabinoids are a specific class of psychoactive compounds present in Indian cannabis (Cannabis sativa), including about sixty different molecules, the most representative being cannabinol, cannabidiol and several isomers of tetrahydrocannabinol. Knowledge of the therapeutic activity of cannabis dates back to the ancient dynasties of China, where, 5,000 years ago, cannabis was used for the treatment of asthma, migraine and some gynaecological disorders. These uses later became so established that, around 1850, cannabis extracts were included in the US Pharmacopaeia and remained there until 1947.
Cannabinoids are known to cause different effects on various systems and/or organs, the most important being on the central nervous system and on the cardiovascular system. These effects include alterations in memory and cognition, euphoria, and sedation. Cannabinoids also increase heart rate and vary systemic arterial pressure. Peripheral effects related to bronchial constriction, immunomodulation, and inflammation have also been observed. The capability of cannabinoids to reduce intraocular pressure and to affect respiratory and endocrine systems is also well documented. See e.g. L. E. Hollister, Health Aspects of Cannabis, Pharmacological Reviews, Vol. 38, pp. 1-20, (1986). More recently, it was found that cannabinoids suppress the cellular and humoral immune responses and exhibit antiinflammatory properties. Wirth et al., Antiinflammatory Properties of Cannabichrome, Life Science, Vol. 26, pp. 1991-1995, (1980).
In spite of the foregoing benefits, the therapeutic use of cannabis is controversial, both due to its relevant psychoactive effects (causing dependence and addiction), and due to manifold side effects that have not yet been completely clarified. Although work in this field has been ongoing since the 1940's, evidence indicating that the peripheral effects of cannabinoids are directly mediated, and not secondary to a CNS effect, has been limited by the lack of receptor characterization, the lack of information concerning an endogenous cannabinoid ligand and, until recently, the lack of receptor subtype selective compounds.
The first cannabinoid receptor was found to be mainly located in the brain, in neural cell lines, and, only to a lesser extent, at the peripheral level. In view of its location, it was called the central receptor (“CB1”). See Matsuda et al., “Structure of a Cannabinoid Receptor and Functional Expression of the Cloned cDNA,” Nature, Vol. 346, pp. 561-564 (1990). The second cannabinoid receptor (“CB2”) was identified in the spleen, and was assumed to modulate the non psychoactive effects of the cannabinoids. See Munro et al., “Molecular Characterization of a Peripheral Receptor for Cannabinoids,” Nature, Vol. 365, pp. 61-65 (1993).
Recently, some compounds have been prepared which are capable of acting as agonists on both the cannabinoid receptors. For example, use of derivatives of dihydroxypyrrole-(1,2,3-d,e)-1,4-benzoxazine in the treatment of glaucoma and the use of derivatives of 1,5-diphenyl-pyrazole as immunomodulators or psychotropic agents in the treatment of various neuropathologies, migraine, epilepsy, glaucoma, etc are known. See U.S. Pat. No. 5,112,820 and EP 576357, respectively. However, because these compounds are active on both the CB1 and CB2 receptor, they can lead to serious psychoactive effects.
The foregoing indications and the preferential localization of the CB2 receptor in the immune system confirms a specific role of CB2 in modulating the immune and antiinflammatory response to stimuli of different sources.
The total size of the patient population suffering from pain is vast (almost 300 million), dominated by those suffering from back pain, osteo-arthritic pain and post-operative pain. Neuropathic pain (associated with neuronal lesions such as those induced by diabetes, HIV, herpes infection, or stroke) occurs with lower, but still substantial prevalence, as does cancer pain.
The pathogenic mechanisms that give rise to pain symptoms can be grouped into two main categories:                those that are components of inflammatory tissue responses (Inflammatory Pain);        those that result from a neuronal lesion of some form (Neuropathic Pain).        
Chronic inflammatory pain consists predominantly of osteoarthritis, chronic low back pain and rheumatoid arthritis. The pain results from acute and on-going injury and/or inflammation. There may be both spontaneous and provoked pain.
There is an underlying pathological hypersensitivity as a result of physiological hyperexcitability and the release of inflammatory mediators which further potentiate this hyperexcitability. CB2 receptors are expressed on inflammatory cells (T cells, B cells, macrophages, mast cells) and mediate immune suppression through inhibition of cellular interaction/inflammatory mediator release. CB2 receptors may also be expressed on sensory nerve terminals and therefore directly inhibit hyperalgesia.
More recently, data suggests a role for CB2 receptor activation in the CNS. Until recently the CB2 receptor was thought to be restricted to the periphery, however emerging data suggests inflammatory pain-mediated induction of CB2 receptor expression in rat spinal cord which coincides with the appearance of activated microglia (Zhang et. al., 2003). Furthermore CB2 receptor agonists have been shown to reduce mechanically evoked responses and wind-up of wide dynamic range neurones in spinal cord dorsal horn in animal models of inflammatory pain (Zhang et. al., 2003, Eur J. Neurosci. 17: 2750-2754, Nackley et. al., 2004, J. Neurophys. 92: 3562-3574, Elmes et. al., 2004, Eur. J. Neurosci. 20: 2311-2320.)
The role of CB2 in immunomodulation, inflammation, osteoporosis, cardiovascular, renal and other disease conditions is now being examined.
Based on the foregoing, there is a need for compounds which have activity against the CB2 receptor. Thus, CB2 modulators are believed to offer an unique approach toward the pharmacotherapy of immune disorders, inflammation, osteoporosis, renal ischemia and other pathophysiological conditions.